1. Field of the Invention
The present invention relates generally to the fields of microRNA molecular biology and cancer. More specifically, the invention relates to the use of microRNA-140-5p as tumor suppressors able to significantly suppress cell proliferation, increase apoptosis, suppress tumor growth and increase sensitivity of chemotherapeutic drugs.
2. Description of the Related Art
RNA interference refers to the process of sequence-specific post-transcriptional gene silencing in animals mediated by short interfering RNAs (siRNAs) (1). The corresponding process in plants is commonly referred to as post-transcriptional gene silencing (PTGS) or RNA silencing and is also referred to as quelling in fungi. The process of post-transcriptional gene silencing is thought to be an evolutionarily-conserved cellular defense mechanism used to prevent the expression of foreign genes and is commonly shared by diverse flora and phyla (2). Such protection from foreign gene expression may have evolved in response to the production of double-stranded RNAs (dsRNAs) derived from viral infection or from the random integration of transposon elements into a host genome via a cellular response that specifically destroys homologous single-stranded RNA of viral genomic RNA.
The presence of dsRNA in cells triggers the RNAi response through a mechanism that has yet to be fully characterized. The presence of long dsRNAs in cells stimulates the activity of a ribonuclease III enzyme referred to as dicer. Dicer is involved in the processing of the dsRNA into short pieces of dsRNA known as short interfering RNAs (siRNAs) (3). Short interfering RNAs derived from dicer activity are typically about 21 to about 23 nucleotides in length and comprise about 19 base pair duplexes (4). Dicer has also been implicated in the excision of 21- and 22-nucleotide small temporal RNAs (stRNAs) from precursor RNA of conserved structure that are implicated in translational control (5). The RNAi response also features an endonuclease complex, commonly referred to as an RNA-induced silencing complex (RISC), which mediates cleavage of single-stranded RNA having sequence complementarity to the antisense strand of the siRNA duplex. Cleavage of the target RNA takes place in the middle of the region complementary to the antisense strand of the siRNA duplex (4). In addition, RNA interference can also involve small RNA (e.g., microRNA, or miRNA) mediated gene silencing, presumably through cellular mechanisms that regulate chromatin structure and thereby prevent transcription of target gene sequences (7-9).
As such, miRNA molecules of the invention can be used to mediate gene silencing via interaction with RNA transcripts or alternately by interaction with particular gene sequences, wherein such interaction results in gene silencing either at the transcriptional or post-transcriptional level. RNAi has been studied in a variety of systems. Fire et al. (1) were the first to observe RNAi in C. elegans. Wianny and Goetz (10) describe RNAi mediated by dsRNA in mouse embryos. Hammond et al. (11) describe RNAi in Drosophila cells transfected with dsRNA. Elbashir et al. (4) describe RNAi induced by introduction of duplexes of synthetic 21-nucleotide RNAs in cultured mammalian cells including human embryonic kidney and HeLa cells.
Small RNAs play an important role in controlling gene expression. Regulation of many developmental processes is controlled by small RNAs. It is now possible to engineer changes in gene expression of plant genes by using transgenic constructs which produce small RNAs in the plant. Small RNAs appear to function by base-pairing to complementary RNA or DNA target sequences. When bound to RNA, small RNAs trigger either RNA cleavage or translational inhibition of the target sequence. When bound to DNA target sequences, it is thought that small RNAs can mediate DNA methylation of the target sequence. The consequence of these events, regardless of the specific mechanism, is that gene expression is inhibited. It is thought that sequence complementarity between small RNAs and their RNA targets helps to determine which mechanism, RNA cleavage or translational inhibition, is employed. It is believed that siRNAs, which are perfectly complementary with their targets, work by RNA cleavage. Some miRNAs have perfect or near-perfect complementarity with their targets, and RNA cleavage has been demonstrated for at least a few of these miRNAs. Other miRNAs have several mismatches with their targets, and apparently inhibit their targets at the translational level.
Without being held to a particular theory on the mechanism of action, a general rule is emerging that perfect or near-perfect complementarity favors RNA cleavage, especially within the first ten nucleotides (counting from the 5′ end of the miRNA), whereas translational inhibition is favored when the miRNA/target duplex contains many mismatches. The apparent exception to this is microRNA-172 (miR172) in plants. One of the targets of miR172 is APETALA2 (AP2), and although miR172 shares near-perfect complementarity with AP2 it appears to cause translational inhibition of AP2 rather than RNA cleavage. MicroRNAs (miRNAs) are noncoding RNAs of about 19 to about 24 nucleotides (nt) in length that have been identified in both animals and plants (12-19). They are processed from longer precursor transcripts that range in size from approximately 70 to 200 nt, and these precursor transcripts have the ability to form stable hairpin structures. In animals, the enzyme involved in processing miRNA precursors is called Dicer, an RNAse III-like protein (5, 20-21). Plants also have a Dicer-like enzyme, DCL1, previously named CARPEL FACTORY/SHORT INTEGUMENTS1/SUSPENSOR1, and recent evidence indicates that it, like Dicer, is involved in processing the hairpin precursors to generate mature miRNAs (18-19).
Furthermore, it is becoming clear from recent work that at least some miRNA hairpin precursors originate as longer polyadenylated transcripts, and several different miRNAs and associated hairpins can be present in a single transcript (12, 22). Recent work has also examined the selection of the miRNA strand from the dsRNA product arising from processing of the hairpin by DICER (23). It appears that the stability, i.e. G:C vs. A:U content and/or mismatches, of the two ends of the processed dsRNA affects the strand selection, with the low stability end being easier to unwind by a helicase activity. The 5′ end strand at the low stability end is incorporated into the RISC complex, while the other strand is degraded.
In animals, there is direct evidence indicating a role for specific miRNAs in development. The lin-4 and let-7 miRNAs in C. elegans have been found to control temporal development, based on the phenotypes generated when the genes producing the lin-4 and let-7 miRNAs are mutated (24-25). In addition, both miRNAs display a temporal expression pattern consistent with their roles in developmental timing. Other animal miRNAs display developmentally regulated patterns of expression, both temporal and tissue-specific (12-15), leading to the hypothesis that miRNAs may, in many cases, be involved in the regulation of important developmental processes. Likewise, in plants, the differential expression patterns of many miRNAs suggests a role in development (16, 18-19), which has now been shown (26).
MicroRNAs appear to regulate target genes by binding to complementary sequences located in the transcripts produced by these genes. In the case of lin-4 and let-7, the target sites are located in the 3′ UTRs of the target mRNAs (24-25, 27-28), and there are several mismatches between the lin-4 and let-7 miRNAs and their target sites. Binding of the lin-4 or let-7 miRNA appears to cause downregulation of steady-state levels of the protein encoded by the target mRNA without affecting the transcript itself (29). On the other hand, recent evidence suggests that miRNAs can, in some cases, cause specific RNA cleavage of the target transcript within the target site, and this cleavage step appears to require 100% complementarity between the miRNA and the target transcript (16, 30), especially within the first ten nucleotides, counting from the 5′ end of the miRNA).
It seems likely that miRNAs can enter at least two pathways of target gene regulation. Protein downregulation when target complementarity is <100%, and RNA cleavage when target complementarity is 100%. MicroRNAs entering the RNA cleavage pathway are analogous to the 21-25 nt short interfering RNAs (siRNAs) generated during RNA interference (RNAi) in animals and posttranscriptional gene silencing (PTGS) in plants (11, 31-33), and likely are incorporated into an RNA-induced silencing complex (RISC) that is similar or identical to that seen for RNAi.
Identifying the targets of miRNAs with bioinformatics has not been successful in animals, and this is probably due to the fact that animal miRNAs have a low degree of complementarity with their targets. On the other hand, bioinformatic approaches have been successfully used to predict targets for plant miRNAs (16, 18, 34), and thus it appears that plant miRNAs have higher overall complementarity with their putative targets than do animal miRNAs. Most of these predicted target transcripts of plant miRNAs encode members of transcription factor families implicated in plant developmental patterning or cell differentiation. Nonetheless, biological function has not been directly demonstrated for any plant miRNA. Although Llave et al. (16) have shown that a transcript for a SCARECROW-like transcription factor is a target of the Arabidopsis miRNA mir171, these studies were performed in a heterologous species and no plant phenotype associated with mir171 was reported.
General categories of sequences of interest for the invention described include, for example, those genes involved in regulating oncogenic processes that are responsible for the initiation, progression or maintenance of increased cell proliferation and/or decreased cell death that are direct or indirect targets of tumor suppressor microRNAs. Target sequences further include coding regions and non-coding regions such as promoters, enhancers, terminators, introns and the like, which may be modified in order to alter the expression of a gene of interest. For example, an intron sequence can be added to the 5′ region to increase the amount of mature message that accumulates (35-36).
In the current invention microRNAs are small-22 nucleotide non-coding RNAs that can bind protein coding mRNAs through complimentary base pairing to mediate mRNA decay or translational repression. Because a single microRNA can bind and silence hundreds of genes across diverse signaling pathways, there is a recognized need in the art to develop microRNAs as powerful therapeutic agents to silence entire disease networks.
The prior art is deficient in the use of the microRNA-140-5p to, inter alia, significantly enhance sensitivity to chemotherapeutic drug as well as provide an alternative or complement to small molecule inhibitor treatment for neuroblastoma and other cancers.